Use the Albright Knot to join Lines of Different Diameters

The Albright Knot is a fishing knot developed specifically for
joining two lines of different diameters without having to use a swivel
or any other hardware at the connection.

If you were to use a swivel here then you'd use a Uni-Knot at the mono side and a Haywire Twist at the single strand wire side, an altogether bulkier arrangement.

With a short wire leader the disturbance of this additional bulk may
well be enough to alarm the fish, particularly in clear water.

Probably best known for joining nylon monofilament or fluorocarbon
line to a single strand wire leader, but also great a great saltwater
fishing knot for connecting one of the very thin spectra or dyneema
braid lines to a thicker nylon monofilament or fluorocarbon leader.

Once snugged up neat and tight it's a small knot which won't catch on the line guides on the cast.

Here's how to tie the Albright Knot

Stage 1

First, double back the end of the single strand wire leader and thread the mono through the eye produced.

Stage 2

Next, start whipping back the mono over itself and the leader.

Stage 3

Then, make about ten turns before tucking the tag end of the mono
through the loop such that it emerges on the same side as it entered.

Stage 4

Finally, lubricate with saliva before smoothly pulling the knot up tight.